Hi Droops
And thank you for asking
Well first of all I shall say that i am a piano player, and that i don't practice the guitare very much
Though i guess i can answer a couple of your questions:
There is a huge difference between simple notes like Mi La Ré Sol and chords E A D G...
Letters represent chords wich is, for exemple the single letter E represents the perfect chord which root note is E. A perfect chord is a chord that has a major third and a perfect fifth. So E would represent the full Mi #Sol Si chord instead of only the root note of it.
Tuning only the root note you are loosing a lot of interesting sounds.
Then the writing goes with altered chors, which could have minor thirds, diminished or augmented fifth, sixth, major or minor seventh, ninth.... and so on.
And then on a single song you can modify the chords, that is for exemple you can change a E- chords (Mi Sol Si) into a E chords (Mi #Sol Si) in some cases.
Or you can change it enirely: for exemple a C-7 chords (Do bMi Sol bSi) into an Eb chord (bMi Sol bSi #Do) still depending on the chord use conditions.
I am sorry but I don't know about Agostini, what about his notation?
Hope that I have been pretty understandable
do not hesitate to ask some more about in case you need or you want to know more